The present invention relates to aircraft control systems and techniques and more particularly, to a system for improving roll response in helicopter control systems.
In fixed wing aircraft control systems of the prior art, a roll command signal and a roll attitude signal are conventionally subtracted from one another to provide an error signal designated as the roll steering signal. The roll steering signal is then coupled as input to an automatic flight control system or exhibited as an error signal so that a pilot may discern when a commanded roll has been achieved. The roll steering command thus causes movement of the aircraft to perform a roll until the roll attitude signal subtracts from the commanded roll to produce the desired attitude and causing the roll steering signal output to be zero.
In a fixed wing aircraft, where the aircraft normally flies a fixed heading with a zero roll attitude, there is very little difficulty in combining the roll attitude and roll command signals to produce the roll steering signal needed to control aircraft roll movement. In some fixed wing configurations, and more particularly in helicopter flight configurations, the aircraft is normally maintained in a non-zero roll attitude even while flying a fixed heading. In this configuration, the roll attitude signal introduces an offset into the roll steering signal of the flight control system which must be compensated for in order to prevent the steering signal from causing a change in aircraft roll attitude when none is required.
In some prior known systems, the offset which would normally be introduced into the roll steering signal of the flight control system has been overcome by use of a limited high pass filter on the roll attitude output signal. In such instances, the high pass filter essentially cuts off any roll attitude signal below predetermined limits by use of a washout filter which enables a response to a commanded roll above those limits. Thus, if the aircraft is required to fly with a non-zero roll attitude, the washout filter will cause the roll attitude to be zero over the long term, yet allow a response to commanded roll during operation of the aircraft to produce the necessary steering signals.
One of the primary disadvantages of the above technique, however, is that for roll attitudes smaller than the limits on the high pass or washout filter, the low frequencies of the roll attitude signal are filtered out, resulting in reduced damping of the roll axis control system and overcompensation in aircraft movement in response to a commanded roll. This results from the presence of a commanded roll signal but no roll attitude signal when the roll attitude signal falls outside the passband of the washout filter. In aircraft which normally maintain a zero roll attitude during pilot flight and autopilot control, the offset signal for non-zero roll attitude is not a significant problem. However, in helicopter systems, where a non-zero roll attitude is the norm, use of the above and similar systems provides undesirable system control and response during aircraft operation.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an aircraft control system having a more natural and smooth response to various aircraft flight conditions.